The present invention relates to an apparatus and method for processing the liquid slag and baghouse dust waste byproducts of an electric arc furnace to recover valuable metal components therefrom. The present invention further relates to an apparatus and method for processing the liquid slag and baghouse dust byproducts of an electric arc furnace to reduce the quantity of waste byproducts.
The worldwide steel industry produces over 750 million tons of crude steel each year. The modern production of steel produces a variety of waste products which are potentially harmful to the environment and the disposal of which is costly. The most predominant of the waste products are slag and steel refining dust.
For every tonne (metric ton or 1000 kilograms) of liquid steel produced by an electric arc furnace (EAF), approximately 110 kg (kilograms) of liquid slag and about 11 kg of steel refining dust is produced. The composition of the slag depends upon the quality of the metallic charge (commercial scrap metal, DRI, etc.) being introduced into the EAF as well as the type of steel being produced and the other components added to the EAF in the production and refining of the steel.
Presently, liquid slag is removed from a steelmaking furnace, such as an electric arc furnace, by a process known as deslagging, in which slag is decanted off of a lower layer of molten metal. During deslagging, residual amounts of valuable metal are inevitably trapped within the liquid slag and subsequently decanted off along with the liquid slag. Metals may also be accidently precluded in the liquid stage during the tapping operation. In an effort to recover such lost metal, liquid slag from the melting furnace has traditionally been discharged into a slag yard where the liquid slag is allowed to cool, and the resulting solid slag is crushed and treated to recover part of the metal value. Some forms of crushed slag, once aged, may be used as a base in road construction. However, slag from an electric arc furnace may not be used in the cement industry because of the less appropriate chemical composition and the increased variability of the EAF slag compared to typical blast furnace slag.
From an environmental standpoint, the handling of slag from an EAF to the slag yard contributes heavily to the pollution of the steelmaking plant and may also be hazardous, depending upon the chemical composition of the slag. When components of the slag are leachable, the slag must be disposed of in a landfill.
Steel refining dust and vaporized chemical components produced during the steel making process provide another waste stream of potential pollutants. Components which escape from the EAF in the form of dust or vapor include zinc, iron, lead, chlorine, magnesium, manganese, and small percentages of other metals, mostly as oxides. Although the dust and vapor (collectively baghouse dust) can be captured with conventional baghouse equipment, the captured baghouse dust must be treated as hazardous waste because of its composition.
Presently, the dust is captured in and collected from the baghouse and subsequently shipped to be processed, to recover mostly zinc and lead. The baghouse dust is considered to be hazardous, due to the potentially hazardous materials it contains, and therefore must always be handled appropriately. Thus, steel production utilizing a typical EAF results in a slag waste stream and a baghouse dust waste stream, both of which contain valuable metallic materials.
What is needed is an apparatus and method for processing liquid slag and baghouse dust from an EAF that recovers metal values from the slag and dust. Further needed is an apparatus and method for processing liquid slag and baghouse dust from an EAF that minimizes pollution at the corresponding steelmaking plant from which the slag and dust are produced by reducing the amount of hazardous materials present in the slag and baghouse dust waste streams. Still further needed is an apparatus and method for processing liquid slag and baghouse dust from an EAF that is energy efficient and provides for safe handling and reduced disposal costs of the slag and dust.
The invention provides an apparatus and method for processing liquid slag and baghouse dust from an EAF that recovers valuable metal components from the slag and dust. The invention also provides an apparatus and method for processing liquid slag and baghouse dust from an EAF that minimizes the pollution of the corresponding steelmaking plant from which the slag and dust are produced. The invention also provides an apparatus and method for processing liquid slag and baghouse dust from an EAF that is energy efficient and provides for safe handling and reduced disposal costs.
The invented apparatus includes a treatment vessel having a moveable lower shell portion for receiving liquid slag. Slag and residual metal values are introduced to the lower shell portion from an electric arc furnace (EAF) during deslagging. In the preferred embodiment, preheated sand (SiO2) is introduced into the liquid slag that is present in the lower shell of the furnace. The preheated sand helps to maintain slag in a liquid state and reduces the amount of energy or heat required to process slag. The treatment vessel also includes an upper shell portion that couples with the moveable lower shell portion prior to activation of the treatment vessel. After adding preheated sand, the upper shell portion is coupled to the movable lower shell portion, and the treatment vessel is heat activated. The treatment vessel of the invention is preferably operated by an AC power source. Alternative embodiments include but are not limited to the use of a DC or a plasma power source for operation of the treatment vessel.
Upon heat activation of the treatment vessel, carbon is introduced into the slag as a reducing agent. Baghouse dust is also introduced to the treatment vessel. During treatment, the molten iron oxide (FeO), manganese oxide (MnO), lead oxide (PbO), and zinc oxide (ZnO) dissolved within the liquid slag and baghouse dust are reduced. The resultant metallic iron and manganese settle to the bottom of the treatment vessel. The more volatile reduced lead and zinc are vaporized within the heat activated treatment vessel.
An off-gas with reduced lead vapor, reduced zinc vapor, and carbon monoxide gas exit the treatment vessel through a flue conduit and thereafter through a flue pipe. The off-gas is injected into a combustion chamber with excess combustion air and reacts with the carbon monoxide to form carbon dioxide. It also oxidizes the lead vapor and zinc vapor. The lead oxide, zinc oxide, and carbon dioxide are cooled in a cooling chamber, and the solid lead oxide and solid zinc oxide are subsequently captured via filtration in a filter chamber. The cleaned, cooled, carbon dioxide is discharged to a stack.
After heat treatment, the treated liquid slag, existing as a top layer within the movable lower shell portion of the treatment vessel, is tapped at a tapping station by decanting the slag away from the reduced molten metal bottom layer.
In one embodiment, the movable lower shell portion of the treatment vessel is equipped with wheels for coupling to a rail to bring the lower shell portion in proximity with the EAF. In an alternative embodiment, the movable lower shell portion of the treatment vessel is moved by a rotatable beam having a support fork, which supports and transports the movable lower shell portion. The EAF and treatment vessel are preferably operated by an AC power source. Alternative embodiments include, but are not limited to, a DC or plasma power source.
The principal object of the present invention is to provide an apparatus and method for processing liquid slag and baghouse dust byproducts from an EAF to recover metal values.
A further object of the invention is to provide an apparatus and method for processing liquid slag and baghouse dust from an EAF that minimizes pollution of the corresponding steelmaking plant from which the slag and dust are produced.
A further object of the invention is to provide an apparatus and method for processing liquid slag and baghouse dust from an EAF that is energy efficient and provides for safe handling and reduced disposal costs.